The invention relates generally to wireless communication systems and, more particularly, to techniques for maintaining synchronization of components within wireless communication systems.
In the past, communication systems used analog technology that did not require precise synchronization between system components, such as a base station and a mobile unit. However, modem communication systems often use modulation techniques, for example code division multiple access (CDMA), that require precise synchronization to allow demodulation of a signal being communicated between the components. In fact, synchronization requirements are often stringent enough that even propagation delays caused by movement of a mobile unit relative to a base station may require resynchronization. While such resynchronization may be performed incrementally while a mobile unit is moving relative to a base station, such resynchronization requires active processing to occur within the mobile unit.
In an effort to extend the battery life of mobile units, mobile units are often deactivated and placed in a dormant, or a quiescent, mode to reduce power consumption and extend battery life. However, when the mobile unit is in a quiescent mode, the active processing that is used during the active mode to maintain synchronization between a base station and the mobile unit is not available. The propagation delay between the base station and the mobile unit may change enough that the traditional technique for resynchronizing the mobile with the base station may be less effective.
FIG. 1 is a diagram illustrating a relationship between a base station and a mobile unit as the mobile unit moves relative to the base station. The horizontal axis denotes time, while the vertical axis denotes energy. At a time TSYS, denoted by impulse 101, if the base station transmits a signal, that signal will not be received by the mobile unit until after a propagation delay 106, illustrated as OFFSET1, has elapsed. After OFFSET1 has elapsed, the mobile unit receives the signal at time TMOBILE1, as denoted by impulse 102. In a real world environment, the signal is affected by reflections and other physical phenomena that cause various components of the signal to arrive at different time offsets, resulting in energy envelope 104, which is spread over a period of time proximate to TMOBILE1 and having its maximal energy at time TMOBILE1.
However, if the mobile unit is placed in a quiescent mode and transported relative to the base station such that the propagation delay between the base station and the mobile unit increases, the time offset between the time at the base station and the time at the mobile unit increases, and the propagation delay 107 is represented as time delay OFFSET2. Thus, after the mobile unit leaves the quiescent mode, the mobile unit receives the signal at time TMOBILE2, as illustrated by impulse 103. As described previously, physical phenomena cause the energy of the signal to be spread over a period of time proximate to time TMOBILE2, denoted by energy envelope 105. While a mobile unit using prior art technology is in the quiescent mode, it is unable to track the change in propagation delay and configure the mobile unit to operate using time delay OFFSET2 rather than time delay OFFSET1. Thus, a mobile unit using prior art technology is not resynchronized after a period of time in the quiescent mode without reacquiring the system.
FIG. 2 is a block diagram illustrating a prior art technique for maintaining synchronization in a mobile unit. A plurality of receiver fingers, including FINGER0, FINGER1, FINGER2, and FINGER3, provide outputs to a multiplexer 201. A FINGERSELECT signal is also provided to multiplexer 201. An output for multiplexer 201 is provided to adder 203. A reference position counter 202 provides an output that is applied as a negative input to adder 203, causing adder 203 to subtract the output of reference position counter 202 from the output of multiplexer 201. The output of adder 203 is provided to clock adjust logic block 204. Clock adjust logic block 204 provides a reference position counter adjustment output to reference position counter 202 in a system time counter adjustment output to system time counter 205. Thus, system time counter 205 is adjusted to provide an output SYS_CNT that is adjusted according to the receiver finger outputs provided to multiplexer 201. However, to maintain synchronization after waking up from the quiescent mode, the entire receiver circuit of FIG. 2 needs to be active. This requires that the receiver fingers, as well as multiplexer 201, reference position counter 202, adder 203, clock adjust logic block 204, and system time counter 205 all remain in an active mode. Thus, all of those elements continue to consume power, thereby defeating the desired benefits of a quiescent mode.
Thus, a more effective technique is needed to maintain synchronization of system components even while one or more system components is in a quiescent mode.